drivers: modem: gsm: Reset attached flag on stop
drivers: modem: gsm: Reset attached flag on stop

The 'attached' flag should be reset on gsm_ppp_stop, or else
some part of the gsm_finalize_connection won't be executed
during the next gsm_ppp_start.

Signed-off-by: Yong Cong Sin <d1951b00420cc51c2da597170bb83d89fa46ae3b@gmail.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr